Soluble activin type IIB receptor improves fracture healing in a closed tibial fracture mouse model

Fractures still present a significant burden to patients due to pain and periods of unproductivity. Numerous growth factors have been identified to regulate bone remodeling. However, to date, only the bone morphogenetic proteins (BMPs) are used to enhance fracture healing in clinical settings. Activins are pleiotropic growth factors belonging to the TGF-β superfamily. We and others have recently shown that treatment with recombinant fusion proteins of activin receptors greatly increases bone mass in different animal models by trapping activins and other ligands thus inhibiting their signaling pathways. However, their effects on fracture healing are less known. Twelve-week old male C57Bl mice were subjected to a standardized, closed tibial fracture model. Animals were divided into control and treatment groups and were administered either PBS control or a soluble activin type IIB receptor (ActRIIB-Fc) intraperitoneally once a week for a duration of two or four weeks. There were no significant differences between the groups at two weeks but we observed a significant increase in callus mineralization in ActRIIB-Fc-treated animals by microcomputed tomography imaging at four weeks. Bone volume per tissue volume was 60%, trabecular number 55% and bone mineral density 60% higher in the 4-week calluses of the ActRIIB-Fc-treated mice (p<0.05 in all). Biomechanical strength of 4-week calluses was also significantly improved by ActRIIB-Fc treatment as stiffness increased by 64% and maximum force by 45% (p<0.05) compared to the PBS-injected controls. These results demonstrate that ActRIIB-Fc treatment significantly improves healing of closed long bone fractures. Our findings support the previous reports of activin receptors increasing bone mass but also demonstrate a novel approach for using ActRIIB-Fc to enhance fracture healing.

骨折仍会给患者带来沉重负担，患者需承受疼痛且有一段时期无法正常劳作。目前已发现多种生长因子可调节骨重塑。但截至目前，临床中仅骨形态发生蛋白（bone morphogenetic proteins, BMPs）被用于促进骨折愈合。激活素（Activins）属于转化生长因子-β（transforming growth factor-β, TGF-β）超家族的多效性生长因子。本团队及其他研究者近期证实，通过捕获激活素及其他配体以抑制其信号通路，使用激活素受体重组融合蛋白治疗，可在多种动物模型中显著提升骨量。然而，目前关于其对骨折愈合的作用尚不明晰。本研究选取12周龄雄性C57Bl小鼠，构建标准化闭合性胫骨骨折模型。将小鼠随机分为对照组与治疗组，分别腹腔注射磷酸盐缓冲液（phosphate buffered saline, PBS）对照试剂或可溶性激活素IIB型受体（activin type IIB receptor, ActRIIB-Fc），每周1次，分别持续2周或4周。造模后2周时，两组间无显著差异；但在造模后4周，通过显微计算机断层扫描（microcomputed tomography, micro-CT）成像观察发现，ActRIIB-Fc治疗组小鼠的骨痂矿化程度显著提升。与对照组相比，ActRIIB-Fc治疗组小鼠造模后4周的骨痂中，单位组织体积骨量提升60%，骨小梁数量增加55%，骨矿密度提升60%（所有指标p值均<0.05）。此外，ActRIIB-Fc治疗还可显著改善造模后4周骨痂的生物力学性能：与PBS注射对照组相比，治疗组骨痂的刚度提升64%，最大载荷增加45%（p<0.05）。上述结果证实，ActRIIB-Fc治疗可显著促进闭合性长骨骨折的愈合。本研究结果既验证了此前关于激活素受体可提升骨量的研究结论，同时也提出了一种利用ActRIIB-Fc促进骨折愈合的全新策略。